The present invention relates to pigments with various advantages over traditional pigment formulations. More specifically, the present invention relates to the use of rare earth manganese oxide compositions for use in pigmentary applications. The rare earth constituent may include yttrium, lanthanum and/or metals in the Lanthanide Series of the Periodic Table. The pigments may be used in plastics, paints, coatings, glass enamels and other materials.
Pigments are widely used in various applications such as paints, inks, plastics, rubbers, ceramics, enamels and glasses. There are various reasons for using inorganic pigments. Pigments may impart coloristic properties and reflectance properties in the visible as well as ultraviolet (UV) and infrared (IR) reflectance spectrums. Their ability to perform this task depends upon which wavelengths are scattered, and which are absorbed when light interacts with them. In order to be suitable in a wide variety of applications, pigments must demonstrate a high degree of light fastness and high temperature stability. A summary of many inorganic pigments and some of their applications can be found in Volume 18 of the Fourth Edition of the Kirk-Othmer Encyclopedia of Chemical Technology, 1996. Classifications of conventional pigments are also given in the publication xe2x80x9cDCMA: Classification and Chemical Description of the Mixed Metal Oxide Inorganic Colored Pigmentsxe2x80x9d, Second Edition, January 1982.
Typical black or dark colored pigments for the plastics industry are based on either carbon black, Crxe2x80x94Fe based hematite blacks, or blacks formulated from various elements, usually containing two or more of the elements Ni, Mn, Cu, Fe and Cr arranged in a spinel type structure. In plastic applications with solar exposure, carbon black pigments absorb light not only in the visible range, but also in the infrared region. This leads to higher temperatures on the exposed colored surface, which may lead to higher cooling costs, or premature failure or degradation of the exposed part. Conventional Cuxe2x80x94Cr oxide compositions, which may contain other additives, do not have particularly good IR reflective properties, but they are useful at moderately elevated temperatures above the exposure temperatures at which carbon black is stable.
Hematite pigments composed of Crxe2x80x94Fe and spinel pigments composed of transition metal oxides often exhibit IR reflective properties. This means that the pigments reflect more light outside the visible range, particularly in the infrared range, than in the visible range. This provides similar color as other pigments, but with a lower temperature when the object is exposed to infrared radiation.
Currently, several of the pigments classified in the Dry Color Manufacturers Association Classifications (DCMA) are used due to their IR reflective performance. The IR reflective performance is determined by the relative amount of reflectance provided in the solar infrared region of the spectrum, i.e. wavelengths of 700 to 2,500 nm compared with the visible region of the spectrum, i.e. wavelengths of 400 to 700 nm. Common black pigments such as carbon black have low reflectances in both regions. To obtain enhanced IR reflectance, inorganic black pigments from the classification DCMA 13-50-9 chrome iron nickel black spinel, or DCMA 3-05-3 chrome green black spinel, are conventionally used for these applications. The pigment DCMA 13-30-3 cobalt chromite green spinel also provides relatively high IR reflectance.
Manganese is used as a chromophore in several mixed metal oxide pigments. In the corundum structure, it is used to form manganese alumina pink corundum (DCMA 3-04-5). It is used as a chromophore in the rutile structure to form brown pigments, such as manganese antimony titanium buff rutile (DCMA 11-20-6), manganese chrome antimony titanium brown rutile (DCMA 11-46-7) and manganese niobium titanium brown rutile (DCMA 11-47-7). It is also a component in many spinel pigments to form brown or black colors. Examples are manganese ferrite black spinel (DCMA 13-41-9), chrome iron manganese brown spinel (DCMA 13-48-7) and chrome manganese zinc brown spinel (DCMA 13-51-7).
The above mentioned pigments may not provide the desired color values, have the desired chemistry and/or provide the desired infrared reflectance properties for many applications. The present invention provides pigments with enhanced color, composition and performance characteristics.
This invention relates to rare earth manganese oxide pigments that can be represented by the formula (RexMn)Oy, where Re represents at least one rare earth element selected from yttrium, lanthanum and elements in the Lanthanide Series of the Periodic Table, x ranges from 0.01 to 99, and y designates the number of oxygen atoms required to maintain electroneutrality. Preferably, x ranges from 0.08 to 12, more preferably from 0.25 to 4, and y ranges from x+1 to 2x+2. These colorants can be used to form colored objects or coatings through their use in applications such as paints, inks, plastics, glasses, ceramics and the like.
An aspect of the present invention is to provide rare earth manganese oxide pigments which impart favorable coloristic properties in the visible spectrum. For example, intense black color may be obtained by these pigments, some of which may also be bluish in shade. The rare earth manganese oxide pigments have shown superior stability and weatherability. These properties are desirable in a wide range of colorant applications.
In accordance with another aspect of the present invention, rare earth manganese oxide pigments provide high infrared reflectance. Yttrium manganese oxide is a particularly preferred pigment which exhibits high IR reflectance. Such high infrared reflectance may result in substantially less heat build up in colored objects using the pigments.
Another aspect of the present invention is to provide pigments which have the ability to change to a different color when exposed to high temperatures. For example, when surfaces containing these materials are subjected to high temperatures by laser marking, legible marks are obtained.
Another aspect of the present invention is to provide pigments which facilitate recycling of colored ceramic or glass articles. When such colored ceramic or glass objects are recycled, less objectionable coloration is passed on to the recycled glass than with conventional black colorants containing cobalt, chromium, nickel and other elements.
Further aspects of the present invention relate to the production of rare earth manganese oxide pigments, substitution of other elements into these pigments, and the use of protective or functional coatings on these pigments in order to enhance their properties.
Another aspect of the present invention is to provide a method of making rare earth manganese oxide pigments. One method includes the steps of mixing rare earth-containing and manganese-containing powders capable of yielding metal oxides, and calcining the mixture.
These and other aspects of the present invention will be more apparent from the following description.